k l'j! 1048 Appendix VL wheel. An endless pulling chain lies in the sprocket j, which is ,;«i keyed to the worm spindle; and the velocity ratio in the example shewn is about 34 : i; for there are 34 teeth in the worm wheel and two threads on the worm, making 17:1, and the radii of j and D are as 2:1; so the total ratio is 17 x 2 = 34 : i. Taking an efficiency of 75 %, the real mechanical advantage is 34 x '75 = 25*5, and a pull of 65 Ibs. by one man will lift 25*5 x 65 = 1660 Ibs. or nearly f of a ton. Referring to the large section in Fig. 942, the thrust of the worm is received in two places, firstly between the collar x on the worm spindle and the steel bush T} and secondly between the bush T and the hard steel pivot u. The former causes the ratchet socket K to bear against two wedges at R, which separate the gun-metal segments s s, and compel them to bind,on the interior of K with a grip proportionate to the weight lifted. As the segments are held to the worm spindle by keys v v on a square on the spindle, the overhaul on the worm is evidently prevented by this frictional grip and the resistance of the pawl w, shewn in lower view; but if the workman desires to lower the weight he can do so by pulling on the lowering side of the chain on j, when the segments s s will slip, the work of over- coming their friction being proportional to the load itself. Thus a light load can be lowered several feet by one vigorous pull on the hand chain, but a heavier load is proportionately resistant. Now, although the frictional resistance is always present, and immediately ready to exert itself towards safety, it is entirely removed on the raising of the load : for the parts E, s, K, and T then revolve as one, the ratchet wheel clicking freely over the pawl; and the only resistance is then the friction of the pivot u, whose diameter is seen to be small.